This disclosure describes a method and an apparatus for improving light exposure, e.g., ultraviolet exposure of photo-curable printing plates, e.g., photopolymer flexographic printing plates, letterpress plates and other polymer printing plates, as well as polymer sleeves and polymer coated printing cylinders. Photo-curable, of course, means curable by photons, e.g., light, e.g., light in the ultraviolet range or some other range.
The term photopolymer plate, or in its shortened form, polymer plate is used herein to refer to any printing plate, cylinder or sleeve that is cured by application of light, such as ultraviolet (UV) radiation, i.e., that is made of or has thereon a photo-curable material such as a photopolymer. The terms “photopolymer” and “polymer” are included because in current practice, the UV curable material is typically made of a polymer. The invention, however, is not limited to use of a polymer material, and the terms “a photopolymer plate,” and “a polymer plate” mean any plate, cylinder or sleeve made of or with any UV curable material thereon, whether polymer or not.
Photopolymer plates have found a broad range of applications. A variety of different methods can be applied for transferring an image for printing, e.g., in the form of imaging data, to a polymer plate. For example, an image mask, which can be a film applied to the surface of the plate while the plate is exposed, or a layer directly on top of the polymer surface is laser ablated layer directly on top of the polymer surface, is placed on top of a polymer sheet.
By a digital plate is meant a plate that is exposed to imaging data by ablating a mask material that is on the plate, e.g., by exposure to laser radiation in an imaging device. The process of producing a digital plate is called a digital process herein.
By a conventional analog plate is meant a plate that is exposed to imaging data by exposing photographic film according to the imaging data, and then using the film to form a mask during exposure to curing radiation. The process of producing a conventional analog plate is called an analog process herein.
Irrespective of the way imaging data is transferred to the plate, the plate needs light, e.g., UV light for curing. Such UV curing is currently done by one of several different methods. After curing, the non-cured portions of the polymer are removed, either using solvents, or by melting the non cured material through heat treatment and absorption with a web.
One UV curing method includes using a bank of UV emitting fluorescence tubes which are placed close to the image mask on top of the polymer plate surface. Although this method produces high quality results, the method requires bulky equipment and a relatively large number of light bulbs leading to high operation costs. Furthermore, due to the bulbs aging differently, homogeneity of the radiation can not be assured, making regular expensive maintenance necessary.
Another UV curing method includes using only a relatively small number of UV light sources—even a single UV source. Having a small number or a single source enables control of the UV light output.
Consider as one example a flat polymer plate, and a high power UV arc lamp with a reflecting mirror above the flat polymer plate. The mirror is designed to direct UV light towards the polymer plate. One undesired property of such and similar arrangements is the relative difficulty to providing homogenous illumination of the polymer of the plate. A typical UV light source such as an arc lamp appears as a point source rather than an extended area source. Hence, relatively complicated and expensive reflector geometries are needed to achieve sufficiently homogenous illumination of the polymer.
One known method of overcoming the problem of achieving homogeneity, is to provide relative motions between the UV light source and the to-be-cured surface. One example of this uses a rotating external drum on the surface of which the plate is placed. With such an arrangement, although each point on the polymer plate receives the same portion of UV light, there are known problems maintaining symmetry of what are called “image shoulders.”
Polymer printing plates are three dimensional, that is, include a depth dimension from the printing surface. Small printing details on the plate's surface carry ink for printing. Consider, for example a halftone dot. In order for the dot not to bend and to be able to withstand the pressure during printing, the halftone dot's surface has to be supported by broader shoulders. With rotational UV exposure, it is difficult to achieve symmetry in such shoulders in the circumference versus axial direction, where the axial direction is the direction of the axis of rotation of the drum.
Thus, today's UV light sources typically require relatively expensive measures for distributing the UV light with a sufficient divergence in order to obtain good support shoulders for the printing surface.